Apple, in a statement to iMore and a few other sites (including Daring Fireball):

“We are extremely excited about the new MacBook Pro, which is the
best pro notebook we’ve ever made. It has the fastest CPU,
graphics, memory, storage and I/O, best display, the innovative
Touch Bar and more. MacBook Pro uses the most advanced
industry-standard connector, USB-C with Thunderbolt 3, to provide
maximum performance, expandability and compatibility.

“We recognize that many users, especially pros, rely on legacy
connectors to get work done today and they face a transition. We
want to help them move to the latest technology and peripherals,
as well as accelerate the growth of this new ecosystem. Through
the end of the year, we are reducing prices on all USB-C and
Thunderbolt 3 peripherals we sell, as well as the prices on
Apple’s USB-C adapters and cables.”

Rene Ritchie:

This price drop on dongles and gear seems to be the
money-where-the-mouth-is to that sentiment. Here’s the new pricing
for Apple’s adapters:

Not included are Apple’s USB-C power adaptors or the USB-C Charge
Cable (2m), but a variety of third-party accessories are also
discounted at Apple Online and at Apple Stores.

Based on the interview and, now, the discount, it feels like Apple
was genuinely shocked by the reaction to the new MacBook Pro.

I think Apple wants to counter the notion that moving to all USB-C is a money grab — that they did it to make money selling adapters. $9 for the USB-C to USB-A adapter is the same price as the Lightning to headphone jack adapter.